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NEET CHEMISTRYThermodynamicsEasy

Question

If enthalpies of formation of C2H4(g)C_{2}H_{4}(g), CO2(g)CO_{2}(g) and H2O(l)H_{2}O(l) at 25∘C25^{\circ}C and 1 atm pressure be 52, –394 and –286 kJ mol⁻¹ respectively, the enthalpy of combustion of C2H4(g)C_{2}H_{4}(g) will be [CBSE PMT 1995; AIIMS 1998; Pb. PMT 1999]:

A

+1412 kJ mol⁻¹

B

–1412 kJ mol⁻¹

C

+141.2 kJ mol⁻¹

D

–141.2 kJ mol⁻¹

Step-by-Step Solution

The enthalpy of combustion (ΔcH∘\Delta_{c}H^{\circ}) is calculated using the standard molar enthalpies of formation (ΔfH∘\Delta_{f}H^{\circ}) of the products and reactants . First, we write the balanced thermochemical equation for the complete combustion of ethene (C2H4C_{2}H_{4}): C2H4(g)+3O2(g)→2CO2(g)+2H2O(l)C_{2}H_{4}(g) + 3O_{2}(g) \rightarrow 2CO_{2}(g) + 2H_{2}O(l) .

According to thermodynamics, the reaction enthalpy is the sum of the enthalpies of the products minus the sum of the enthalpies of the reactants: ΔrH∘=[2×ΔfH∘(CO2,g)+2×ΔfH∘(H2O,l)]−[ΔfH∘(C2H4,g)+3×ΔfH∘(O2,g)]\Delta_{r}H^{\circ} = [2 \times \Delta_{f}H^{\circ}(CO_{2}, g) + 2 \times \Delta_{f}H^{\circ}(H_{2}O, l)] - [\Delta_{f}H^{\circ}(C_{2}H_{4}, g) + 3 \times \Delta_{f}H^{\circ}(O_{2}, g)] .

By convention, the standard enthalpy of formation for an element in its reference state, such as O2(g)O_{2}(g), is taken as zero .

Substituting the values provided in the problem: ΔcH∘=[2(−394 kJ mol−1)+2(−286 kJ mol−1)]−[52 kJ mol−1+3(0)]\Delta_{c}H^{\circ} = [2(-394 \text{ kJ mol}^{-1}) + 2(-286 \text{ kJ mol}^{-1})] - [52 \text{ kJ mol}^{-1} + 3(0)] ΔcH∘=[−788−572]−52\Delta_{c}H^{\circ} = [-788 - 572] - 52 ΔcH∘=−1360−52=−1412 kJ mol−1\Delta_{c}H^{\circ} = -1360 - 52 = -1412 \text{ kJ mol}^{-1}.

The negative sign confirms that combustion is an exothermic process . This result matches Option B.

Exam Context & Concepts Covered

This question aligns with the NEET CHEMISTRY syllabus, specifically targeting concepts from Thermodynamics. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

CHEMISTRYThermodynamicsenthalpiesformationpressurerespectivelyenthalpy

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